Available by going to Settings > Battery > Battery Health, the section provides a look at the current maximum capacity of your battery relative to when it was new.

It also will let you know if the battery is currently supporting peak performance capability. Anyone with an iPhone 6, iPhone 6 Plus, iPhone SE, iPhone 6s, iPhone 6s Plus, iPhone 7 and iPhone 7 Plus can disable the power management option designed to prevent unexpected shutdowns as a battery ages.